Microprocessors simplify humidity measurements

Wet-bulb and dry-bulb thermistors are interfaced directly with a microprocessor without the need for a separate A/D converter or other complex analog signal conditioning circuitry to provide a simple and reliable yet flexible and cost-effective approach to measuring humidity. Two such circuits are described along with the digital signal processing algorithms that help extend the range and linearize the response of the sensors. With the first approach, the thermistors become part of a dual resistor-controlled-oscillator circuit; oscillator frequencies are mapped into dry-bulb and wet-bulb temperatures. For the second approach, the thermistors reside in a pulsewidth-modulation circuit; pulsewidths are mapped into dry-bulb and wet-bulb temperatures. The transfer function and sensitivity coefficients for each circuit are calculated. Overall measurement system accuracy is limited by the uncertainties in the wet-bulb and dry-bulb sensors and not by thermal drift, component aging, variations in power supply voltages, or nonlinearities in other circuit components.